vorbisdec: support freeing partially allocated contexts.

Fixes null pointer derefernces

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
oggdec: resync from the last page.

Previously we re synced from where we where which cam lead
to loosing pages.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vorbisdec: handle midstream parameter changes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
oggdec: Leave treatment of serial changes to the decoder.

Attempting to re-parse the headers at demuxer level is a
pandora box the way its done currently.

This allows full reconfiguration of vorbis streams

Fixes Ticket2117
Fixes Ticket2121

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>